This invention generally relates to nondestructive evaluation and, more particularly, to measuring small changes occurring within a volume of material as an effect on the propagation of waves within the volume. This invention is the result of a contract with the Department of Energy (Contract No. W-7405-ENG-36).
A significant problem in the field of nondestructive evaluation (NDE) is detecting progressive small changes in material properties that can lead to catastrophic events, e.g., the fatigue failure of bridges or airplane wings. Traditionally, the use of acoustic signals to characterize elastic properties of engineering materials has relied primarily of direct-path measurements of travel time and wave amplitude of the first arriving signal. Travel time ultrasonic tomography can provide redundant signals and yield images of velocity structures within materials, but the spatial extent and resolution of these images are limited by the number of sources and receivers used to generate and collect the necessary waveform data. The precision with which travel-time measurements can be made is limited by the timing precision of the recording instruments used and by the noise in the signals.
In areas that use seismic signals to characterize physical properties of rocks, low resolution imaging of volumes with few source-receiver pairs can be achieved by using the redundancy properties of coda waves, i.e., late-arriving scattered acoustic waves, whereby the late-arriving waves have sampled larger regions of material surrounding the source and receiver. Coda waves generated by natural earthquakes have been used to determine material properties in various regions of the earth. Progressive phase delays between coda waves generated by nearly identical pairs of natural earthquakes (called doublets) which occur at different times in the same location have been analyzed to determine local changes in seismic velocity occurring in the time interval between earthquakes.
There has been substantial analytical work done using natural seismic sources, e.g., earthquakes. See, e.g., Poupinet et al., "Monitoring Velocity Variations in the Crust Using Earthquake Doublets: An Application to the Calaveras Fault, California," J. Geophys. Res., 89:5719-5731 (1984). There has, however, been little extension of this work because (1) naturally occurring doublets are hard to find; (2) even when natural doublets are found, there is no guarantee that the source locations are absolutely identical for both events, leading to ambiguous interpretations of measured phase delays; and (3) the algorithm used is not robust and requires substantial user interaction due to inherent instabilities and ambiguities in phase-delay estimates.
The present invention has recognized that the doublet method has important applications to high sensitivity NDE of commercial products, both on a production basis and over extended periods of time, since the technique is capable of analyzing a bulk volume of a work piece. Accordingly, it is an object of the present invention to detect small changes in bulk material properties as an early indication of changes in material strength and fatigue life.
Another object of the present invention is to detect changes occurring over volumes of material rather than along a single line-of-sight path.
One other object of the present invention is to eliminate source location ambiguities.
Still another object of the present invention is to improve the stability of the analysis algorithm using the doublet response detected at the receiver transducers.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.